[127.03] The ``Aerogel'' Model for the Origin of the Short-Lived Radionuclides in the Early Solar System

Isotopic analyses of meteorites have revealed that our Solar
System contained a number of live short-lived radionuclides
at its birth. These include {}41{\rm Ca} (t1/2 =
0.10 Myr), {}36{\rm Cl} (0.30 Myr), {}26{\rm Al}
(0.71 Myr), {}10{\rm Be} (1.5 Myr), {}60{\rm Fe}
(1.5 Myr), {}53{\rm Mn} (3.7 Myr), {}107{\rm Pd}
(6.5 Myr), {}129{\rm I} (15.7 Myr), and {}182{\rm
Hf} (9 Myr). The radionuclide {}10{\rm Be}, which must
be created by spallation reactions, is known to be decoupled
in meteorites from the other radionuclides, and must have a
separate origin that predates the Solar System. Its origin
has been attributed to trapping of {}10{\rm Be}
Galactic cosmic rays in the Sun's molecular cloud core
(Desch et al.\ 2004; ApJ 602, 528). The most plausible
explanation for the other radionuclides is a nearby
supernova. Most models of injection of supernova
radioactivities into the early Solar System hypothesize that
the supernova triggered the collapse of the Sun's molecular
cloud core. Chevalier (2000; ApJ 538, L151) has suggested
instead that the supernova occurred after the Sun's
protoplanetary disk had formed, and at a distance of < 1
pc, in analogy to the proplyds observed in the Orion Nebula
only a few tenths of a parsec from \theta1 Ori C. We
use meteoritical and astrophysical evidence to argue that
this is by far the most plausible scenario for how the Solar
System acquired its short-lived radionuclides. We
hypothesize that radionuclides in the supernova ejecta
condensed into grains which were then injected into our
protoplanetary disk; there they were stopped like dust
grains lodged in aerogel. Because of the proximity of the
disk to the supernova, a key prediction of this ``aerogel''
model is the presence of very short-lived radionuclides in
the early Solar System (< 104 yr). We discuss the
recent, tentative evidence for live {}63{\rm Ni}
(t1/2 = 101 yr) in the early Solar System (Luck et al.\
2003; GCA 67, 143) in this context, and discuss the effect
of the injected radioactivities on the ionization state of
the solar nebula.